Electrophotographic recording member



Jan. 3, 1967 5. J. SCHOENFELD 3,295,967

ELEGTROPHOTOGRAPHIC RECORDING MEMBER Filed Sept. 5, 1963 5 6 4 PHOTOSENSiTIVE COAT 2 j"-"I'f-'.'""'.'",'-'T 5 CONDUCTIVE BARRIER COAT"'JLCONDUCTIVE BACK 3 COAT FIG. I

IO b 2 I 0 lo 10B. 2 3: Z3 {5 lo LU II l I I I I I I l 0% IO 4o so so soREL. HUM.@ 72 F United States Patent 3,295,967 ELECTROPHOTOGRAPHICRECORDING MEMBER Samuel J. Schoenfeld, Neenah, Wis., assignor toKimberly- Clark Corporation, Neenah, Wis., a corporation of DelawareFiled Sept. 3, 1963, Ser. No. 305,979 12 Claims. (Cl. 961.5)

This invention relates to an electrophotographic recording member and amethod of producing the same; more specifically, the invention relatesto new and improved electrostatic recording elements made desirablyelectrically conductive by the incorporation of a coating between anon-metallic base of high electrical resistance and a photosensitivecoating.

Electrophotographic recording members having a nonmetallic base such assheets of paper coated with a photosensitive material are commonlyemployed by electrostatically charging the coating in the dark and thenexposing selected portions of the charged surface to light; a latentelectrostatic image is then formed on the surface of the recordingmember and this is subsequently developed by dusting an oppositelycharged powder on the coating in the dark. The powder adheres to theareas of higher electrostatic charge, that is, the areas of the surfacewhich were not illuminated to a high degree, and the powder does notadhere in significant quantity to the illuminated areas. The powderapplied to develop the latent image is usually fusible and anessentially permanent image is achieved without the necessity of atransfer process.

Important to the attainment of adequate prints is the requirement thatthe areas of the charged recording element which are exposed to light bereadily discharged. Incomplete discharge results in a fuzzy print, yetthe recording member itself must have sufiicient electrical resistancethat the recording member will hold a suitable charge in the dark for areasonable time. Additionally, since the recording member inconventional practice becomes the print, coated material, when the baseis a paper sheet, must exhibit all the qualities of a good copying aper.

The electrical conductivity of such recording members has posed adifiicult problem in the art. Customarily, at high humidities the memberresponds reasonably well but at low humiditiesbelow about 25% attemperatures of about 70 F.the member tends to become dry, lowering theelectrical conductivity and inhibiting the discharge of lighted areas.Various expedients have been employed in the art to overcome thisdefect. For example, the members have been humidified before use,additionally, paper sheets have been treated to increase theirconductivity. The former operation is cumbersome and the latter hasinvolved relatively expensive compounds.

A primaryobject of this invention is to provide a novelelectrophotographic member which is useful over a wide range ofhumidities under normal temperature conditions.

An important object of this invention is to provide on a non-metallicinsulating base a conductive coating to which a photosensitive coatingis to be subsequently applied to thereby provide an electrophotographicmemher, and which member is useful at low relative humidities undernormal temperature conditions of about 70 F.

Another object of this invention is to provide an electrophotographicmember in the form of a composite which includes an electricallyconductive coating on a non-metallic base and which coating resists thepenetration of organic solvents upon application of an organicsolventdispersed photosensitive material to the conducting coat ing.

Yet another object of the invention is to provide a novel method ofproducing an electrophotographic memher.

I have found that a sheet very suitable in all other characteristics andwhich is useful in the low humidity rangesdown to at least 15% relativehumidity at about 70 F.may be produced by incorporating in anintermediate coating, between the non-metallic base and thephotosensitive upper layer, a coating containing relatively specificcomponents. These components include hydrated silicic acid and ahygroscopic hydrated inorganic salt. The coating containing thesecomponents is well bonded both to the base and the upper photosensitivelayer and is not materially subject to deterioration in the practice ofthe printing process, or storage, before or after the printing. Y

The coating composition which forms the intermediate stratum ispreferably an aqueous mineral coating composition of the typeessentially common to the paper coating art but modified to include thenoted specific components and to attain desired properties. In thislatter connection I prefer to usually employ in such compositions arelatively high binder content, about 50% to 70% by weight of the solidsof the composition, to assure against powdering, solvent penetration,and lack of binder strength; most suitably, the binder content is 55% to60% of the total solids and may be any conventional binder such asstarch, casein, or other protein.

Further, in the practice of the invention, I prefer to form the hydratedsilicic acid in the com-position in situ by employing an excess of anacidic reacting hygroscopic hydrated inorganic salt; however, ifdesired, the silicic acid may be precipitated from a silicate as the gelby other components such as methyl alcohol, mineral acids and amines,for example, methylamine. In any event, the pH is further lowered beyondthat sufficient to provide for the hydrated gelatinous silicic acidformation by the addition of the hydrated inorganic salt.

I have found that, while a pH of the coating composition as low as 4 maybe utilized, it is preferable to maintain the pH of the coatingcomposition as applied to the paper to between about 7.2 and 7.8. Theupper limit of pH, I have found to be about 8.2 in order to secureadequate gelation and formation of the hydrated silicic acid. While a pHof the final composition on the acid side is useful, difiiculty may behad with the storage of such papers over a long period of time.

The non-metallic base and the photosensitive coating may be materialscommonly employed in the art. Preferably, the base is paper as alreadynoted, but it may be cloth, leather and the like. Suitably, thephotosensitive coating is a photoconductive zinc oxide dispersed in abinder material of high electrical resistance, higher than that of thenon-metallic base; other photosensitive components such as zinc sulfide,cadmium sulfide, and various selenides and oxides, particularly thoseapplied from organic solvent systems, are useful.

In general, I prefer to mix the complete coating composition minus thehydrated inorganic salt; then the salt is added as a solution to gel thesilicate; and this salt is added in sufiicient quantity to reduce the pHto at least 8.0 and preferably to between 7.2 and 7.8. Where a gellantother than the inorganic hydrated salt is employed, the gellant is addedfirst to give the composition a pH of about 8.2, and after gelationtakes place, the hydrated salt is mixed in. The same final pH rangesthen hold for the completed composition.

The invention will be more fully understood by referenceto the followingdetailed examples and accompanying drawings wherein:

FIG. 1 represents a sectional view of an electrophotographic member inaccordance with the invention; and

FIG. 2 is a graph illustrating resistance characteristics versushumidity of a conductive sheet prepared in accordance with theinvention.

Example 1 A paper base comprised of about 55% by weight bleached sulfiteand 45% kraft having a mixed Canadian standard freeness of about 350 anda basis weight of about 44 pounds (25 X 38 x 500 sheets) and a lateralresistivity in excess of ohms per square at RH, 70 F., was coated withthe following composition:

Water suflicient to make solids of about 40%.

The above noted composition was formulated by combining the clay, starchand the hexametaphospha-te in slurry form with the water and cooked at atemperature of 190 F. The cooking time was 30 minutes. While the slurrywas hot, the sodium silicate was added while mixing in a ribbon-typekneader. A Z-bar kneader or Lightnin mixer has also been found suitable.To the mixed slurry the magnesium chloride solution was then addedslowly. During this addition there is a marked thickening of the batchand a tendency to appear grainy. This graininess disappears as the batchis mixed and the components are thoroughly interspersed. Subsequent tothe incorporation of the magnesium chloride solution the soap is addeddry. The viscosity of the resulting coating composition is approximately60 as measured on the Brookfield viscometer with a No. 6 spindle at 100r.p.m. at 50 C. The pH of the composition was about 7:5.

The composition was applied initially to one side of the paper sheet andthe dry coating weight was about four pounds per ream (25 x 38 x 500sheets). While application of a coating to the second side is notnecessary for this purpose of achieving electrical conductivity, acoating on the second side is desired to avoid any curling of the finalpro-duct. Accordingly, a coating of about the same weight andcomposition in this instance is applied to the second side of the sheet.The coating composition is applied to the sheet by conventional rollcoating procedure and thereafter the sheet is dried and supercalendered.

In the course of the formulation of the composition, the magnesiumchloride reacts with the highly alkaline sodium silicate to produce ahydrated silicic acid and this silica gel readily retains water. Themagnesium chloride in the above formulation is itself added insutficient amount to attain the pH of 7.5 in order that magnesiumchloride hydrate is present in the final product. The dried web issupercalendered at low pressures sufiicient to attain a gloss of about30 to simply smooth the coating on the sheet; the hydrated silicic acidand magnesium chloride hydrate are substantially unaffected thereby. Iprefer low supercalender pressures to avoid forced impregnation of thesheet and to achieve a relatively stiff sheet which I have found moreuseful in the copying machines. The lateral resistance of thisconductively coated sheet was 1.8 x 10 ohms/sq. at 50% RH at 70 F. and5.2 x 10 ohms/sq. at 10% RH at 70 F.

A second coating comprising a dispersion of a photosensitive componentin an insulating binder is then prepared as follows:

Parts by weight, lbs. 0

The above mixture is ground in a ball mill for several hours and is thenready for application as the upper layer coat. This composition isapplied by rod coat procedure to the conductive coating on one side ofthe sheet and dried. The thickness of the photoconductive layer is about0.45 mil.

The silicone resin employed is a film-forming commercial productmarketed as GE Silicone Resin SR-82, a product of the General ElectricCompany of Pittsfield, Massachusetts. The zinc oxide is a French ProcessPhctox 801, a product of New Jersey Zinc Sales Company, Inc., of NewYork, NY.

Paper produced in accordance with this example was then subjected to anelectrostatic charge in the dark and subsequently exposed to visiblelight radiation to effect discharge.

The following table illustrates the apparent surface voltage aftercharging, a dark decay period and after exposure to 3% foot candles forvarious lengths of time at 10% and 50% RH at 70 F.

Seconds V0 1 1 2 10% RH 676 541 91 73 21 0 50% RH 540 540 88 62 5. 9 0

1 Vn=apparent surface voltage after charge. 1 V =apparent surfacevoltage after 6.25 seconds dark decay.

As will be noted, the dark decay rate is small though slightly greaterat 10% RH; also, the light decay results are entirely acceptable.

Additionally, sheets of the electrostatic recording medium were subjectto regular printing operations and the prints were found to be excellentfor their intended purpose.

Example 2 A paper base having a furnish as described in connection withExample 1 and a base weight of about 46 pounds (25 x 38 x 500 sheets)was coated with the following formulation:

Water addition 575 In the formulation of the above composition clay,starch, soap and water are cooked in conventional manner at about 185 F.for about 20 minutes. The mixture is then cooled to approximately F. andthe sodium silicatae solution added. When the sodium silicate has beenwell mixed, the aluminum nitrate in solution is added slowly withcontinuous agitation. Again during the addition the mass thickensconsiderably and gel formation in the form of silicic acid takes place.With continued agitation the composition breaks down to yield a smoothcoating composition easily applied by roll, rod or blade. Suchcomposition has a solids content of about 30%, a Brookfield viscosity(same conditions as Example 1) of 54.7, and a pH of about 7.0.

The web is coated on both sides, dried and supercalendered, as describedin Example 1. The coating weight in this instance was about 3 pounds perream per side. The electrical resistance of the coated sheet at 14%relative humidity at 75 F. was 8.2 ohms/square, measured laterally. Thissheet was then coated on one side with a zinc oxide coating as describedin connection with Example 1 and was found thoroughly satisfactory inuse in conventional electrostatic copying equipment.

In the above described electrostatic recording members the base sheetcocated with the electrically conductive layer serves as an excellentbarrier to prevent penetration of the solvent of the zinc oxide coatingduring application of this latter coating.

The functioning of the hydrates apparently is as follows: as thehumidity becomes lower, the hydrated silicic acid which is electricallyconductive tends to give up a part of its water and in turn takes waterfrom the hydrated inorganic salt. Since the latter salt is hygroscopiceven at these low humidities, it takes water from the atmosphere tomaintain its essentially hydrated condition. It is to be noted that thehydrated inorganic salt in the crystal form is not itself electricallyconductive and does not contribute to the electrical conductivity of thesheet in any significant manner at higher relative humidities.

Example 3 The following coating composition has been found useful as aback coating for the second side of the sheet opposite thephotoconductive coating. It is formulated and applied in the same manneras previously described. The utility is as a conductive coating as wellas a backing coating.

Soap

Such composition, when applied as described in previous examples andutilized as the conductive barrier coating treatment for thephotoconductive coat, has an electrical resistance of about 10ohms/square at 14% RH at 75 F.

This latter formulation, for the purposes of enhancing its utility as aback coating, may have added thereto about 170 pounds of tack inhibitorsuch as Carbowax 4000 (a polyethylene glycol) to eliminate any tack inthis high binder coating. Carbowax 4000 is a product of Carbide andCarbon Chemical Company and is a non-hygroscopic waxy solid of amolecular weight of about 4000 having a saybolt viscosity in seconds at210 F. of about 375. When such component is utilized, it is preferableto add the same dry to the complete coating formulation set forth aboveand to knead the mass until the polyethylene glycol is dissolved. Such acoating composition has a solids content of approximately 37% and aviscosity (measured as previously described) on the Brookfieldviscometer of approximately 75.

Referring now to the drawings, the numeral 1 designates a non-metallicbase sheet such as paper of relatively poor electrically conductingproperties in excess of 10 ohms/square. The numeral 2 indicates aconductive barrier such as has been previously described. The coatingindicated at 3 is essentially a coating for the purpose of inhibitingsheet curl and may have the same constitution as coating 2 or mayvarying in specific composition as previously noted. The photosensitivecoating is desig- 6 nated by the numeral 4 and comprises aphotoconductive material indicated at 5 dispersed in an insulatingbinder 6. Binder 6 as is conventional in the art has a high electricalvolume resistivity of about 10 to 10 ohms cm.

The paper with the intermediate layer 2 thereon, for the purposes ofthis invention, should have an electrical resistance of between about 10to 10 ohms/square at relative humidities of 10-15% at about 70 F. It isto be noted that repeated tests utilizing the foregoing formulations andvariations thereof as examplified by FIG. 2 of the drawings haveprovided sheets which meet this requirement. It is further to be noted,as is obvious, that the quantity of intermediate coating compositionapplied to the sheet will affect the electrical conductivity through thesheet and will, to some extent, affect the lateral conductivity.However, for application of photoconductive compositions having anorganic solvent system, the minimum quantity of coating applied is animportant factor in inhibiting solvent penetration. Such penetration isundesirable as it leads to inferior photoconductive coating lay. Ingeneral, coating weights per side with these aqueous mineralcompositions of about 3 to 10 pounds are suitable. Additionally, thesheets should not be excessively conductive or they will fail to retainsufiicient of the charge and imperfect prints will result when used in acopying machine. I

It is my opinion that the efiiciency of the defined coating compositionsand electrostatic recording members is due in large part to thecooperative relationship between the inorganic salts present and thesilica gel, the silica gel functioning not only to maintain the memberin suitable condition for receipt of charge and discharge, but also toprevent washout of the soluble salts during the coating application.Soluble salts present such as sodium chloride contribute in some measureto conductivity through the action of the hydrated components.

With respect to the coating composition, I may, as already noted, employto 70% binder in the intermediate coating although I prefer about tobased on the dry coating Weight. However, for backing coatings wheresolvent holdout is not as important, this range may be from 40% to andis preferably for economic reasons from 42% to 50% by weight on the drycoating. The backing coating is then less liable to blistering as thecoating is more open and moisture escapes more readily when the paper isused in copying machines. Additionally, it is to be noted that I preferthat the silicate employed in the barrier coating composition suitablyforms about 9% to 11% by weight dry 'basis, and the magnesium chloridehydrate about 11% to 13% by weight of the coating composition (drybasis), the balance being a pigment such as clay to facilitatecomposition drying. For a back coating composition the preferred rangeof silicate and hydrate are somewhat greater, about 11 to 14% and 12 to15% (dry basis by weight) respectively, due to the decreased bindercontent of this coating.

It will be understood that this invention is susceptible to modificationin order to adapt to different usages and conditions and, accordingly,it is desired to comprehend such modifications within the invention asmay fall within the scope of the appended claims.

What is claimed is:

1. An electrophotographic member comprising a composite having anon-metallic base of high electrical resistance, a coating on said basefor increasing the electrical conductivity of the composite, saidcoating comprising gelatinous hydrated silicic acid and a hygroscopichydrated inorganic salt, said coating covered by a photosensitivestratum comprising a photoconductive component and an insulating binder,said component bein-g dispersed in said binder and said binder having anelectrical resistance greater than that of the base.

2. An electrophotographic :member as claimed in claim 1 and in which thehygroscopic hydrated inorganic salt is selected from the groupconsisting of hydrated magnesium chloride and hydrated aluminum nitrate.

3. An electrophotographic member as claimed in claim 1 and in which thehygroscopic hydrated inorganic salt is hydrated magnesium chloride.

4. An electrophotographic member as claimed in claim 1 and in which thehygroscopic hydrated inorganic salt is hydrated aluminum nitrate.

5. An electrophotographic member comprising a paper base of highelectrical resistivity, an upper layer comprising a zinc oxidephotoconductor and an electrically insulating film-forming binder inWhich the zinc oxide photoconductor is uniformly dispersed, said binderhaving an electrical resistance greater than that of the base andphotoconductor, and an intermediate layer bonded to said base and saidupper layer and comprising a coating having therein gelatinous hydratedsilicic acid and a hygroscopic hydrated inorganic salt.

6. An eleotrophotographic member as claimed in claim 5 and in which thehygroscopic hydrated inorganic salt is selected from the groupconsisting of hydrated magnesium chloride and hydrated aluminum nitrate.

7. An electrophotographic member as claimed in claim 5 and in which thehygroscopic hydrated inorganic salt is hydrated magnesium chloride.

8. In the manufacture of coated paper for electrophotographic purposeswherein an overlying photosensitive stratum is applied, the step, priorto the application of the photosensitive stratum, of applying an aqueousmineral conductive coating containing gelatinous hydrated silicic acidand a hygroscopic hydrated inorganic salt, said coating compositionhaving a pH of from 4 to about 8.0.

9. In the manufacture of coated paper for electrophotographic purposeswherein an overlying photosensitive stratum is applied, the step, priorto the application 8 of the photosensitive stratum, of applying anaqueous mineral conductive coating containing gelatinous hydratedsilicic acid and a hydroscopic hydrated inorganic salt, said coatingcomposition having a pH of from about 7.2 to 7.8.

10. In the manufacture of coated paper for electrophotographic purposeswherein an overlying photosensitive stratum is applied, the step, priorto the application of the photosensitive stratum, of applying an aqueousmineral conductive coating containing gelatinous hydrated silicic acidand a hygroscopic hydrated inorganic salt, said coating compositionhaving a pH of from about 7.2 to 7.8 and a binder content based on thetotal solids of the composition of between about and by weight based onthe total solids of the composition.

11. An electrophotographic member comprising a paper base of highelectrical resistivity, an upper layer comprising a zinc oxidephotoconductor and an electrically insulating film-forming binder inwhich the zinc oxide photoconductor is uniformly dispersed, said binderhaving an electrical resistance greater than that of the base andphotoconductor, and an intermediate layer bonded to said base and saidupper layer and comprising a coating having therein gelatinous hydratedsilicic acid and a hy-groscopic hydrated inorganic salt, and a backingcoating on the second side of the base, said backing coating alsocontaining gelatinous hydrated silicic acid and a hygroscopic hydratedinorganic salt.

12. An electrop'hotographic member as claimed in claim 11 and in whichthe backing coating contains an agent to inhibit tack and blocking.

No references cited.

NORMAN G. TORCHIN, Primary Examiner.

1. AN ELECTROPHOTOGRAPHIC MEMBER COMPRISING A COMPOSITE HAVING ANON-METALLIC BASE OF HIGH ELECTRICAL RESISTANCE, A COATING ON SAID BASEFOR INCREASING THE ELECTRICAL CONDUCTIVITY OF THE COMPOSIT, SAID COATINGCOMPRISING GELATINOUS HYDRATED SILICIC ACID AND A HYGROSCOPIC HYDRATEDINORGANIC SALT, SAID COATING COVERED BY A PHOTOSENSITIVE STRATUMCOMPRISING A PHOTOCONDUCTIVE COMPONENT AND AN INSULATING BINDER, SAIDCOMPONENT BEING DISPERSED IN SAID BINDER AND SAID BINDER HAVING ANELECTRICAL RESISTANCE GREATER THAN THAT OF THE BASE.